In recent years, technologies such as HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access) that improve the transmission rate of data have been proposed.
Here, according to R99 (Release 99) standard, data is transmitted by using DCH (Dedicated Channel). For example, in the DCH, the size of PDU: Protocol Data Unit (for example, RLC PDU) is the first size (for example, 42 octets).
On the other hand, in HSDPA, data is transmitted by using HS-DSCH (High Speed Downlink Shared Channel). For example, in HS-DSCH, the size of PDU (for example, RLC PDU) is the second size (82 octets). Note that the second size is larger than the first size.
Thus, by using the PDU of the second size, which is larger than the first size, the data transmission rate can be improved without extending the possible range of the sequence numbers allocated to the PDU (for example, Patent Document 1).
A receiver is configured to accumulate PDUs in a buffer, and to transmit acknowledge information (ACK/NACK) to a transmitter indicating whether or not the receiver receives the PDU successfully. By using the PDU that are retransmitted from the transmitter, the receiver recovers an error of the PDU that could not be received by the receiver.
As described above, if the size of the PDU changes, the size of the PDUs accumulated in the buffer becomes different from the size of the PDU retransmitted from the transmitter. As a result, because the error occurring in a radio interval cannot be recovered, the PDUs accumulated in the buffer are discarded. Thus, due to a change in the size of the PDU, the already transmitted PDU becomes useless, which results in a loss of data.
Similarly, when the size of the PDU is changed, transmitter needs to discard the PDU that has not acquired the acknowledge information indicating that the receiver has been successful in receiving the PDU, which results in a loss of data.